I would say something in the digital arena. Like transparent said - a digital chip maybe - right now, they have the "hard-drive" players that hold the large amount of music. Also, I can see these players of the future having a color display screen, which would mean all the cover art and stuff that comes with the CD's could be displayed, and maybe even more - like high quality videos to carry around. I still see technology with an infinite future - things that are amazing seem to happen everyday.
MP3's are a start, but they are just deploying players that can hold 100's of hours of music. For rasons mentioned before - I eventually think will will be digital - nothing to scratch, nothing to skip, easier to move between songs, and I like the thought of my entire CD collection fitting in my pocket.
There are plenty of purists who still hang onto LP's, thus, in x-years from now, CD's will still be around. But as technological advances occur, and the digital format evolves - there are way too many positives to ignore it.